Disk drive storage system are commonly used for storing data in electronic products including digital cameras, music players, computer systems, and the like. As shown in FIG. 1, a disk drive 10 comprises a magnetic recording medium, in the form of a disk or platter 12 having a hub 13 and a magnetic read/write transducer 14, commonly referred to as a read/write head. The read/write head 14 is attached to or formed integrally with a suspension arm 15 suspended over the disk 12 and affixed to a rotary actuator arm 16. A structural arm 18, fixed to a platform 20 is pivotably connected to the actuator arm 16 at a pivot joint 22. A voice coil motor 24 drives the actuator arm 16 to position the head 14 over a selected position on the disk 12 for writing data to the disk 12 or reading data from the disk 12.
A spindle motor (not shown) rotates the disk 12 while the moving air generated by the rotating disk, in conjunction with the physical features of the suspension arm 15, lifts the read/write head 14 away from the platter 12, allowing the head to glide or fly on a cushion of air slightly above an upper surface of the disk 12. The flying height of the read/write head over the disk surface is typically less than a micron.
An arm electronics module 30, electrically connected to the head 14 by flexible conductive leads 32, includes circuits operative during the data read and write operations. Although the module 30 may be mounted anywhere in the disk drive 10, a location proximate the head 14 minimizes signal losses and induced noise in the read and write head signals. A preferred mounting location for the module 30 comprises a side surface of the structural arm 18 as shown in FIG. 1.
As shown in a partial cross-section and partial block diagram of FIG. 2, the disk 12 comprises a substrate 50 and a thin film 52 disposed thereover. Write current supplied to a write head 14A alters magnetic domains of ferromagnetic material in the thin film 52 to store the data bits as magnetic transitions.
Data bits to be written to the disk 12 are supplied by a processing device 60 to a data write circuit 62 where the data bits are formatted and error detection/correction information appended thereto. To write data bits, the voice coil motor 18 moves the suspension arm 16 to a desired radial position above the surface of the disk 12 while the spindle motor rotates the disk 12 to move a circumferential region to be written under the write head 14A.
A write driver 66A of a preamplifier 66 (in one embodiment disposed within the electronics module 30) supplies a programmed write current (in certain applications between about 10 mA and 70 mA) to the write head 14A responsive to a signal representative of the data to be written to the disk 12 supplied by a data write circuit 62.
During a write operation, the write current supplied by the write driver 66A to the write head 14A (magnetically coupled to a magnetically permeable core not shown) creates a magnetic field that extends from the core across an air gap between the write head 14A and the disk 12. The magnetic field alters a region of magnetic domains in the thin film 52 to store data bits.
The direction of the magnetic field generated by the write head 14A, and thus the direction of the altered magnetic domains, is responsive to the direction of current flow through the write head 14A. Current supplied in a first direction through the write head 14A causes the domains to align in a first direction (representing a date 0 for example) and current supplied in a second direction (representing a data 1 for example) causes the domains to align in a second direction.
In the read mode, transitions between adjacent domains are detected to determine the stored data bit. During read operations a read head 14B (typically comprising a magneto-resistive (MR) sensor, a giant magnetoresistive sensor (GMR), or a tunneling magnetoresistive head (TMR)) senses the magnetic field transitions in the thin film 52 to detect the stored data bits.
The suspension arm 16 moves while the disk 12 rotates to position a read head 14B above a magnetized region to be read. A DC (direct current) bias voltage of between about 0.025V and about 0.3V is supplied to the read head 14B by a read circuit 66B of the preamplifier 66. Magnetic domains in the thin film 52 passing under the read head 14B alter a resistance of the magneto-resistive material, imposing an AC (alternating current) component on the DC bias voltage. The AC component representing the read data bits has a relatively small magnitude (e.g., several millivolts) with respect to the DC bias voltage.
The read circuit 66B amplifies the read signals and supplies the amplified voltages to a data read circuit 72 where error detection/correction is performed, and the data bits detected. The data bits are supplied to a user interface 74, such as an interface to a computer or data processing device (e.g., SATA, SCSI, SAS, PCMCIA interfaces). Amplification of the signal in the read circuit 66B reduces the relative noise contribution of the data read circuit 72 when the signal is processed through the circuit 72.
In other data storage systems, the head 14 can operate with different types of storage media (not shown in the Figures) comprising, for example, a rigid magnetic disk, a flexible magnetic disk, magnetic tape, a magneto-optical disk, and the like.
To increase storage capacity, a disk drive may comprise a plurality of stacked parallel disks 12. A read/write head is associated with each surface of each disk to write data to and read data from a top and bottom surface of each disk.
Ideally, the write current supplied to the write head 14A is a purely differential write current. Thus, the current sources that supply the write current are designed to produce symmetrical voltage/current swings during write operations. However, as is known by those skilled in the art, circuit topology, manufacturing processes and parasitic circuit elements introduce aberrations in the write circuit that generate a common mode signal. Also, when the current sources and other components operative during write operations are turned off, for example at the beginning of a read operation, common mode currents (voltages) are generated and introduced into the write head 14A. Common mode voltage spikes of several volts are not atypical at write turn-off.
The magnetoresistive material of the read head 14B cannot tolerate excessive voltages. In contrast, the thin-film inductor of the write head 14A requires relatively large magnitude, sub-nanosecond voltage swings to write data to high data-rate disk drive systems. Due to the close spacing of the read head 14B and the write head 14A, asymmetrical (i.e., common mode) voltages supplied to the write head 14A may be coupled to the read head 14B, potentially damaging or destroying the read head 14B.
Write current is typically supplied by the write driver 66A, comprising a plurality of current sources each controlled by an emitter follower transistor operative with a corresponding bias resistor. One technique for turning off the write current after a data write operation simultaneously deactivates the current sources and opens switches that control a connection between the bias resistor and its corresponding emitter follower transistor. Since the current sources turn off faster than the resistor-controlled switches open, a common mode voltage nearly equal to the supply voltage is imposed across the write head 14A and therefore coupled to the proximate read head 14B. State-of-the-art read heads (e.g., GMR and TMR heads) cannot tolerate such voltage transients.
As one skilled in the art will appreciate, precise turn-off timing is difficult to achieve and becoming more difficult as larger capacity and faster devices are required by today's hard disk drives. Switching misalignment of only tens of picoseconds can cause the switches to deliver a sufficient charge to swing the voltage on the write head 14A from the positive supply voltage to the negative supply voltage.
It is known to use clamps and catches to slowly lower the voltage in response to a change in a switch position. But these techniques are limited to controlling power down excursions and therefore are not usable during power-up, which can also generate common mode voltages in the write head 14A.
It is also known to slowly increase (and decrease) the circuit bias voltages while the switching devices are in the proper state for biasing. However, timing of the bias voltage increase (and decrease) relative to turning on (off) the switching devices must be carefully controlled as the switching devices can deliver a substantial charge. This technique is further discouraged by fast-mode switching requirements of state-of-the-art heads, requiring a fast bias voltage response during both power up and power down. Thus, a need exists for apparatus and methods for controlling common mode voltages in disk drives and/or other storage devices.